This invention relates to apparatus for melting and dispensing thermoplastic materials, and more particularly to the control of the fluid pump utilized in such apparatus.
The majority of pumps used in hot-melt plastic dispensing units are similar to the pumps used for painting and other fluid dispensing applications in which a large air piston is connected to a small fluid piston to obtain a multiple ratio between the amount of air pressure required and the amount of fluid pressure developed. Some systems utilize single-acting pumps, that is, pumps which make use of only one-half of the piston movement for pumping action. Other systems utilize double-acting pumps which make use of both halves of the piston stroke to accomplish pumping action.
It will be appreciated that the double-acting pump can more closely approach a steady flow of fluid material as compared to the single-acting pump in which no fluid is pumped for half of the piston stroke. It has become a problem, however, even in double-acting pumps, to maintain a steady flow of fluid material because of the piston action at the end points of piston travel when the pump reverses its direction of stroke.
It has been conventional in the art for double-acting pumps used in hot-melt and painting applications to utilize a mechanical toggle-type of switching system for reversing the pump. These mechanical toggles present manufacturing difficulties and also operational problems. In operation the volume and pressure of the fluid at the applicator head is noticably affected when the double-acting piston pump reaches its points of reversal, at each end of the stroke. As the pump piston approaches one end of its travel and begins to activate the toggle switching mechanism, the piston is working against a pressure force which it did not have during the earlier part of its travel. This additional force requires a change in work produced at the outlet at this point of piston travel. As the piston continues to bear against the toggle switching mechanism and starts to move it, additional force is encountered by the piston, and, depending upon the force required to operate the toggle mechanism the piston will move somewhat slower. The resistance offered by the toggle is not uniform, but increases as the toggle approaches dead center and varies thereafter in inverse fashion from the dead center position to the end of stroke. The additional resistance force of the toggle acting against the piston produces a diminishing of pressure available for work from the time the piston first reaches the toggle until it has moved the toggle to cause reversal.
Conventional toggle mechanisms are relatively expensive to build and require careful machining, lapping of parts and additional detail for proper operation. The use of toggle mechanisms also results in a long pump because the additional length of the pump taken up for the toggle is not added to the stroke.